1. Field of the Invention
The present invention relates to a semiconductor device isolation, and more particularly, to a method of dual bird's beak local-oxidation-of-silicon (LOCOS) isolation.
2. Discussion of the Related Art
In implementing a semiconductor device on a semiconductor substrate such as a silicon substrate, device isolation is preferentially performed. A LOCOS of thermal oxidation is an example of semiconductor device isolation. A LOCOS device isolation layer includes “bird's beaks” at both ends of the device isolation layer. The size of the bird's beak affects the operation of a device.
High-voltage or high-power devices are substantially less affected by a size of the bird's beak. Yet, such devices include logic components operating at a relatively low voltage. For the low-voltage logic components, LOCOS having a small bird's beak is preferred. Instead of LOCOS, shallow-trench isolation is substantially used for reducing a design rule having a size of 0.25 μm or below.
However, high-voltage or high-power devices prefer LOCOS that provides a device isolation layer having a smoother end to mitigate an electric field applied to a junction. By reducing the electric field effect applied to an edge of the LOCOS layer, a device becomes more stable in terms of reliability and its safe-operating-area characteristics and exhibits a lower leakage current.
In high-voltage or high-power process technology, a logic component is basically isolated by LOCOS. The high-voltage or high-power process technology plays a role in protecting the logic component if a great electric field is applied to a drain extended region of a high-voltage device, e.g., DMOS or LDMOS.
For instance, the LOCOS device isolation process includes steps of forming a pad oxide layer for device isolation, depositing a pad nitride layer or a layer for a hard mask for device isolation, forming a hard mask for device isolation by patterning, and removing the nitride pad layer used as the hard mask by wet etching. After completion of the LOCOS device isolation process, a transistor is fabricated.
Hence, the LOCOS process provides the same-sized bird's beak regardless of a high-voltage device or a low-voltage device. Since the device isolation is performed mainly for the high-voltage device, the bird's beak on the edge of the LOCOS layer is formed to be smooth and large. Hence, a design rule for a low-voltage logic circuit is considerably raised.
However, since the logic devices as the logic components are formed together with the high-voltage or high-power device, when integrating the high-voltage and logic devices together, a fabricating process meeting the two requirements is needed. The design rule needs to be reduced for cost-effective logic devices. For high-voltage devices, the LOCOS layer having a smooth bird's beak is formed to fabricate a stable high-voltage device.